Tightly packed (5)
I believe the answer is:
dense
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tightly packed</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for dense that I've seen before include "Thick, impenetrable" , "Obtuse - impenetrable" , "Heavy" , "Stupid; compacted" , "Thick; stupid" .)